1. Field of the Invention
The present invention relates to a heat treatment apparatus which performs a heat treatment in a magnetic field. Particularly, the invention relates to a heat treatment apparatus which applies a heat treatment in a high magnetic field to a magnetic material such as an MR film or a GMR film.
2. Description of the Related Art
A magnetic film such as a thin film of an Fe--Ni alloy or the like formed on a substrate, by sputtering or the like, which is a magnetic material used for a magnetic head or the like can exhibit its magnetic properties by subjecting it to a heat treatment in a high magnetic field.
For this purpose, there is conventionally proposed a heat treatment apparatus in which an electric furnace, an induction heating furnace or the like is disposed to apply a heat treatment in a magnetic field formed with electromagnets. A schematic configuration of a typical conventional heat treatment apparatus is illustrated in FIG. 7.
As shown in FIG. 7, the heat treatment apparatus 1A has a cylindrical vacuum vessel 2, and magnetic field generating means 20 arranged outside the vacuum vessel 2. In the vacuum vessel 2, a holder supporting unit 4 is attached to an upper part thereof. A holder 3 holding an object to be heat-treated such as a magnetic material is charged into the interior by this supporting unit 4 and held. The magnetic field generating means 20 is provided with a pair of electromagnets 21 arranged oppositely outside the vacuum vessel 2, and the electromagnet 21 has a magnetic core 22 and a coil 23.
Heating means 100 is provided between the outer surface of the vacuum vessel 2 and the end face of the magnetic core 22 of the electromagnet 21. Usually, the heating means 100 is spaced apart from the outer surface of the vacuum vessel 2 by a prescribed distance, and comprises electric heaters 101 arranged so as to surround the outer periphery of the vacuum vessel 2. The electric heater 101 is formed, for example as shown in FIG. 7, by providing, for example spiral grooves 103 on the inner periphery of a heater support 102 made of bricks or ceramics arranged so as to surround the vacuum vessel 2, the inner periphery facing the outer periphery of the vacuum vessel. A heating wire such as a nickel-chromium wire 104 is positioned in each of these grooves 103. Heat insulators 105 such as alumina felt or bricks are arranged on the outer periphery of the heater support 102 so that heat of the heating means 100 is not transferred to the electromagnets 21.
In the conventional heat treatment apparatus, however, since the heating means 100 with the structure as mentioned above is provided between the outer periphery of the vacuum vessel 2 and the magnetic core of each of the electromagnets 21, the distance between the magnetic cores 22 and 22 of the pair of electromagnets 21 is inevitably large. The distance (L) between the inner surface of the vacuum vessel 2 and the magnetic core 22 is usually within a range of from 135 to 250 mm. If the vacuum vessel 2 is assumed to have an outside diameter (D1) of 240 mm, the distance (LO) between the magnetic cores 22 of the electromagnets 21 would therefore be within a range of from 520 to 700 mm.
In order to achieve a uniform magnetic field and a prescribed magnetic field strength, therefore, the electromagnet composing the magnetic field generating means 20 must necessarily be large in size, and the current fed to the electromagnet 21 must be increased. The increase in weight and current resulting from the use of larger electromagnets leads to the necessity to provide a larger equipment space and a larger power supplying facility.
Adoption of a larger electromagnet leads to a larger overall construction of the heat treatment apparatus, and considerably increases the equipment cost and the running cost.
Furthermore, with the aforementioned heating means, in which bricks or the like are used as heat insulators, dust is produced, and this causes problems when using in a clean room.